Some optical imaging systems include a transmissive or a reflective imager panel, also referred to as a light valve or light valve array, which imposes an image on a light beam. Transmissive light valves are typically translucent and allow light to pass through. Reflective light valves, on the other hand, reflect only selected portions of the input beam to form an image. Reflective light valves provide important advantages, as controlling circuitry may be placed behind the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. New potentially inexpensive and compact liquid crystal display (LCD) and digital light processor (DLP) projector configurations are becoming possible through the use of reflective microdisplays as the imager panel.
Projection systems typically use one or three imager panels. Single panel systems are often operated in a mode that is referred to as “field sequential color”. In such a mode, a white light source is used to produce a beam of white light. A color filter, such as a color wheel, selectively filters the light so that only one color is incident on the imager panel at any one time. The imager panel is controlled to impose the image corresponding to incident color. The filter cycles through the colors, typically red, green and blue, with the imager panel synchronized to impose the image of the incident color of light. The net effect seen by the viewer is a full color image, even though only a single color is projected at any one time. The use of such a filter, however, generally results in about two-thirds of the light being discarded at any one time, since the lamp supplies white light but the imager panel is illuminated with only one color. Thus, single panel systems suffer from reduced efficiency.
One way common way of addressing this problem is to use three imager panels, each one associated with its own color band. As a result, each image panel can be illuminated continuously, and so the efficiency is increased relative to the single panel system. Three panel systems, on the other hand, are much more complex, require precise alignment among all the imager panels, and are more expensive due to the larger parts inventory. Thus, there remains a desire to obtain high efficiency operation of a simple projection system that uses fewer parts.